The inherent advantages of employing natural gas as a fuel are becoming more and more apparent in light of increasingly restrictive environmental regulations. One area of significant potential is the use of liquefied natural gas as a transportation fuel. Specific areas of the transportation sector where such use is particularly appealing includes the automotive, trucking and rail sectors. A major problem in employing liquefied natural gas is localized availability and the lack of a delivery network analogous to that existing for conventional liquid fuels. A second problem area is that the development of process technology for natural gas liquefaction has generally focused on world-scale plants capable of producing greater than 400 MMSCF/D of liquefied product. The current invention provides a method and apparatus for producing relatively small volumes of liquefied natural gas on a more localized basis.
It is common practice in the art of processing natural gas to subject the gas to cryogenic treatment to separate hydrocarbons having a molecular weight higher than methane (C.sub.2 +) from the natural gas thereby producing a pipeline gas predominating in methane and a C.sub.2 + stream useful for other purposes. Frequently, the C.sub.2 + stream will be separated into individual component streams, for example, C.sub.2, C.sub.3, C.sub.4 and C.sub.5 +. One such separation process which has received wide spread application in natural gas plants is the turbo expansion process. This process is illustrated in FIG. 1 and is characterized by its overall simplicity.
Representative process conditions for the turbo expansion process are as follows. Feed gas is delivered to the process via conduit 1 at a pressure of about 500 to about 1500 psig and a temperature of about 60.degree. to about 100.degree. F. Water is then removed from the stream by dehydrater 50 thereby producing via conduit 3 a gas product possessing a dewpoint of about or less than -100.degree. F. Conduit 3 is connected to feed gas cooler 52 wherein the stream is cooled via indirect contact with cold residue gas introduced via conduit 23 thereby producing a heated residue stream via conduit 24 and a two-phase cooled stream via conduit 5. The resulting two-phase stream produced via conduit 5 is then routed to separator 54 from which is produced a liquid stream via conduit 7 which is then introduced as feed into a separator stabilizer or demethanizer, a term that is used herein interchangeably by those skilled in the art. The separator stabilizer or demethanizer is a fractionating column with respect to the liquid stream injected via conduit 7. The column possesses both rectifying and stripping sections. The methane-rich vapor stream produced from separator 54 is routed via conduit 9 to turbo expander 58 wherein the stream undergoes pressure reduction and associated cooling thereupon producing energy and a two phase mixture containing appreciable quantities of liquid (ex., 20 wt % liquid) via conduit 11. This two phase mixture is typically at a pressure of about 50 to about 600 psig and a temperature of about 0.degree. to about -180.degree. F. The two phase mixture is introduced into upper section of the stabilizer 56 where it contacts rising vapors and undergoes phase separation thereby producing a methane-rich vapor via conduit 23 and a liquid stream which functions as a reflux stream in the column. Liquid leaves the stabilizer via conduit 13 and is fed to reboiler 60. Heat to the reboiler is usually provided via a heating medium which may be a feed gas side stream. The heating medium is delivered via conduit 17 and returned via conduit 16. Vapor is produced from the reboiler and returned to the stripping section of the stabilizer via conduit 21. A C.sub.2 + rich liquid product is produced from the reboiler 60 via conduit 15.
As previously noted, vapor which has also been previously referred to as a cold condensate gas is produced from the top of the stabilizer via conduit 23 and flows to the feed gas cooler 52 wherein this stream is warmed and produced via conduit 24. The contents of this conduit may then be employed as fuel via conduit 25 and/or recompressed via flow through conduit 27 to recompressor 62 wherein power generated via turbo expander 58 is used to compress the gas. This compressed gas is produced via conduit 29. If additional compression is required, additional power may be provided to compressor 62 or the contents of conduit 29 as noted in FIG. 1 may be routed to a separate compressor 64 thereby producing via conduit 31 a gas stream at a greater pressure. Although C.sub.2 + recoveries will be dependant on design parameters and desired products, ethane recoveries of up to 90% and propane recoveries of 70 to 99% are possible. Butane and heavier component recoveries of 95 to 100% are possible.
As previously noted, the liquefaction of natural gas is frequently conducted for transport and storage purposes. The primary reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design. For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate the pipeline under a substantially constant and high load factor. Often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks when demand exceeds supply, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand, thereby enabling future peaks in demand to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a supply source to market when the source and market are separated by great distances and a pipeline is not available or is not practical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significant reduce the specific volume of the gas which in turn requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to -240.degree. F. to -260.degree. F. where it possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior an for the liquefaction of natural gas or the like in which the gas is liquefied by sequentially passing natural gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, and methane or with mixed refrigerants of given compositions. The refrigerants are frequently arranged in a cascaded manner and each refrigerant is employed in a closed refrigeration cycle. Further cooling of the liquid is possible by expanding the liquefied natural gas to atmospheric pressure in one or more expansion stages. In each stage, the liquefied gas is flashed to a lower pressure thereby producing a two-phase gas-liquid mixture at a significantly lower temperature. The liquid is recovered and may again be flashed. In this manner, the liquefied gas is further cooled to a storage or transport temperature suitable for liquefied gas storage at near-atmospheric pressure. In this expansion to near-atmospheric pressure, significant volumes of liquefied gas are flashed. The flashed vapors from the expansion stages are generally collected and recycled for liquefaction or utilized as fuel gas for power generation.